Tuning fork assembly including driving and pick-up coils



Feb. 7, 1961 w. J. HOLT, JR

TUNING FORK ASSEMBLY INCLUDING DRIVING AND PICK-UP COILS Filed April 22, 1957 2 Sheets-Sheet 1 Fig.5

INVENTOR William J. Holt, Jr.

ATTORNEY Feb. 7, 1961 w. J. HOLT, JR 2,971,104

TUNING FORK ASSEMBLY INCLUDING DRIVING AND PICK-UP COILS Filed April 22, 1957 2 Sheets-Sheet 2 BY flxwxmv ATTORNEY United States Patent TUNING FORK ASSEMBLY INCLUDING DRIVING AND PICK-UP COILS Filed Apr. 22, 1957, Ser. No. 654,193

12 Claims. (Cl. 310-47) This invention relates to the construction and arrange ment of tuning forks.

It is an object of this invention to provide a new and improved tuning fork.

It is an object of this invention to provide a new and improved tuning fork assembly which is simpler and more economical to manufacture than thosoe of prior art, and yet which exhibits superior performance characteristics.

Another object of this invention is to provide a tuning fork assembly having a higher Q.

Another object of this invention is to provide a tuning fork assembly having an improved magnetic circuit.

Another object of this invention is to provide a tuning fork assembly wherein the fork mounting frame is metal.

These and other objects are effected by this invention as will be apparent from the following description taken in accordance with the accompanying drawings, forming a part of this application, in which:

Figure l is an elevational view of a fork assembly;

Figure 2 is a center longitudinal section view of the tuning fork assembly of Figure 1;

Figure 3 is a transverse section view taken along lines 33 of Figure 2;

Figure 4 is an exploded View showing all of the parts of a complete tuning fork assembly except the outer housing, or can;

Figure 5 is a plane section view of the top of the tuning fork assembly taken along lines 55 of Figure 2;

Figure 6 is an elevational view of an embodiment of a completed tuning fork assembly.

Figure 7 is a center longitudinal section view of the tuning fork assembly of Figure 6.

Figure 8 is a transverse section view of the tuning fork housing assembly taken along lines 8-8 of Figure 7.

Figure 9 is a transverse section view through one of the coils and core to show the magnetic lines of force.

Referring now to the drawings and particularly to Figure 2 and 4, it will be seen that the tuning fork assembly comprises a base plug 11, a resilient ring 12, a fork 13, a driving coil 14, a pick-up coil 15, a two piece housing for the fork and coils comprising substantially identical complementary left and right hand parts 16, 17, a support ring made up of substantially identical complementary left and right hand parts 18, 19, a pair of skirts 20, and an outer housing, or cam 21.

The fork 13 is preferably of the solid base type made from a metal bar of magnetic material of rectangular cross-section although other type bar stock and bases may be employed. A deep U-shaped slot is formed in the bar so that the metal remaining at the slot sides constitutes the fork tines 22. The housing parts 16, 17, are made of sheet metal which has been stamped and formed. Each housing part has a lower channel portion 23 merging with an enlarged upper channel portion 24. The lower channel portion 23 is comprised of a channel bottom 25 and two sides which extend substantially perpencompleted tuning dicular to the plane of the channel bottom. In the preferred embodiment, a flange 26 extends outwardly from each side which is in a plane of the said channel bottom. Punched holes 27 may be provided in the channel bottom for the coil lead wires. The flanges 26 are adapted to butt the base portion of the tuning fork and may be, of course, adapted to fit any shaped base. The upper channel portion 24 is shaped similarly to the lower chan nel portion having a channel bottom which is wide enough to accommodate the coils 14, 15 between two sides which extend substantially perpendicular to the plane of the channel bottom. Each channel side wall is provided with a pair of spaced semicircular notches 28 to provide an air gap between the housings 16, 17 and the core magnets 29 which extend through the coils 14, 15. A notched flap 30 is provided at the upper end of each upper channel bottom and extends perpendicular to the channel bottorn. The notched flaps 30 of each housing are formed so that they conjunctively form an end Wall for the upper channel of the housing parts 16, 17. The drive coil 14 and pick-up coil 15, which are identical, are in the form of short cylindrical bobbins, each having a single winding thereon and are mounted on magnetic cores or pins 29 which have north and south poles.

The support ring when assembled, has the shape of a wide band which has a pair of identical parallel plane surfaces 31 on opposite sides thereof, and is otherwise generally circular. The ring has a pair of inwardly extending indentations, or dimples 32 which are symmetrical with respect to a plane perpendicular to a diameter joining said flat surfaces 31. The ring is separated into two identical parts 18, 19, one lying on each side of the last mentioned plane. The resilient ring 12 is a resilient means such as a band, and is designed to fit closely over the tuning fork assembly support ring 18, 19. The base plug 11 is a metal ring 33 having a disc 34 of non-conduction material fixed to its inner periphery with plug pins 35 fixed to the disc. The base plug outer diameter is such that the plug will fit snugly inside the lower end portion of the outer housing, or can 21. The skirts 20 are identical halves of a thin metal cylinder each having an inwardly extending dimple 36 in its upper portion. The outer housing, or can 21, is simply a thin metal cylinder having one end rounded and closed. As shown in Figure 6 the cam may be provided with a pair of inwardly extending indentations, or dimples 37, which are disposed to coact with the dimples 32 on the support ring 18, 19, if it is desired to eliminate the skirts 20.

In assembly, the coils 14, 15 and magnetic core or pins 29 are positioned, one above the other, between the upper channel portion 24 of the housing parts 16, 17 and the notches 28 so that the north pole of one magnetic core is positioned on the same side as the south pole of the other magnetic core and, the other side has the respective south and north pole of the magnetic cores. An air gap 38 should extend between the sides of the housings 16, 17 and the notched flaps 30 as shown in Figure 8. The notches 28' in the sides of the housings form an air gap 39 about the core pins 29 and the tines 22 of the tuning fork form an air gap 40 between the tines and the core pins 29. The coil leads are then brought down through the channels or holes 27. The volume enclosed by the upper channel portion of the housing parts is then impregnated with an epoxy resin 41, as for example, the one known in the trade as Scotch Cast or any other suitable binder material. The coils 14, 15 and their core pins 29, and the upper channel portion of the housing parts 16, 17, now form a rigid unitary structure. Next the fork 13 is positioned between the lower channels 23 and flanges 26 of the housings and the flanges 26 are spot welded to the tuning fork base. The upper channels and side thereof, of each housing part extend between the fork tines 22 and the marginal edges of the sides are aligned and closely adjacent, but are not in contact wlth each other. The notched flap 30 of each housing part co-operate with each other in a closely adjacent interlocking fashion, but are not in contact as shown in Figure 5. The interlocking and the binder also provide a strong lateral and longitudinal support for the housing parts.

The two parts of the support ring 18, 19, are next assembled to form the support ring with its longitudinal axis substantially coincident with the central longitudinal axis of the fork 13, and with the plane surface portions 31 in contact with the back surfaces of the lower channel portion 23 of the housings 16, 17. The plane surface portions of the support ring parts are then spot welded to the back surfaces of said respective lower channels. The ring dimensions are made such that the marginal edges of the ring parts at the dimples 32 are closely adjacent, but not in contact. At this point of construction, only the base plug 11, the resilient ring 12, the skirts Z and the can 21 remain to be added to the assembly. Itis at this point of construction that the tuning fork may be dynamically balanced. Balancing is accomplished by applying an input of a standard or reference frequency to the driving coil 14 and removing or adding material on the fork tines 22 until the output from the pick-up coil 15 is the desired value. After balancing the fork, the leads 42 of the coils, which were brought down through the channels or holes 27 prior to the impregnating step, are soldered to appropriate ones of the base plug pins 35. Next, the

resilient ring 12 is fitted over the support ring 18, 19,

skirts 20 are positioned and the assembly is placed in the can 21 with the dimples as of the skirts 2t) aligned in cooperative relation with the support ring dimples 32, and finally the base plug 11 is fitted into the open end of the can, forming a closure for same, and is spot welded thereto.

The rigid mounting of the tuning fork base to the housings and the resilient mounting of the tuning fork and housing in the can provides an increased efficiency for the tuning fork assembly in that there is little energy loss between the tuning fork and housings due to the rigid mounting which eliminates any relative movement between the housing and fork which would cause a dampening effect and it also provides a constant relative positionment of the tuning fork with respect to the housing. The resilient mounting of the tuning fork and housing in the can provides a floating mounting so that the energy loss from the tuning fork and housing to the can is minimized. The Q of the tuning fork is a function of the molecular friction properties of the metal itself, the amount of 'unbalance present and the damping imposed by the magnetic fields and air and therefore it will be appreciated that by minimizing the damping imposed by the unit, the Q is improved.

The operation of the tuning fork will now be explained. Figure 9 shows the improved magnetic circuit wherein the magnetic lines of force between the north and south poles of the magnetic core pin 29 are increased and concentrated through the metal housing. This is accomplished by decreasing the effective air gap between the north and south poles which decreases the reluctance and increases the numer of magnetic lines of force and since the magnetic lines of force will take the path of least resistance, the lines of force are concentrated around the metal housing. The decrease in reluctance and increase of magnetic lines of force greatly improves the generated output voltage of the tuning fork. In other words, the metal frame provides a more positive flux path for the lines of force which increases the eificiency of the fork and also acts to shield the pick-up coil from the drive coil. This provides a lower output when the drive frequency is off resonance (due to transformer action), and a higher output at the resonant frequency. The metal frame allows use of magnetic cores of less magnetic strength for a given outpuL voltage (assuring constant drive voltage) than the magnetic strength of cores required for a non-metalic frame. This, in turn, decreases the magnetic damping and the Q will be increased. If an alternating current is now applied to the driving coil 14, it will produce an alternating magnetic field around the coil 14. This alternating magnetic field alternately increases or decreases the number of magnetic lines of force of the magnetic core pin 29 which in turn causes the tines to vibrate at frequency of the applied alternating current. For example, if during the positive half cycle of current, the number of magnetic lines of force is increased, the tines will be drawn inward toward the poles of the core due to the increased lines of force. During the negative half cycle of current, the number of magnetic lines of force is decreased and therefore the tines move further away from the poles of the core and their momentum will carry them past their original positions, the succeeding cycle of alternating current continues the vibrations. The core pin 29 in the pick-up coil 15 has a magnetic circuit as described with respect to the core pin 29 in driving coil 14 and it will be apparent that the vibration causes the tines to cut through the magnetic lines of force about the core pin 29 of the pick-up coil 15 which will, in turn, induce an alternating current in the pick-up coil in a manner well-known.

It will be apparent from the foregoing that the arrangement and construction of the tuning fork assembly of my invention provides an extremely rugged and compact unit. The fork and coil housing frame may be made of stamped and formed sheet metal pieces with a resulting high degree of economy. This metal frame also acts to improve the magnetic circuit of the assembly by providing a better flux path, resulting in a high output for the unit. The construction further provides for a floating mounting which allows only a low dissipation of energy and also acts as a shock mounting which will minimize external vibration effects. The construction and arrangement additionally permits precise and convenient dynamic balancing of the fork under actual operating conditions with the fork and coils and their housing completely assembled in final form. This latter is true because the fork tines are exposed in the final assembly so'that test equipment can be readily applied and the metal necessary for precise dynamic balance can be readily removed or added on the fork tines.

While I have shown my invention in only one form, it will be apparent to those skilled in the art that it is not so limited, but is susceptible of various changes and modifications without departing from the spirit thereof. For example, in a manner well-known to those skilled in the art, the cores may be of soft iron which is alternately magnetized. Also a pulsating direct current may be employed as a driving force, if desired.

It will be apparent to those skilled in the art that various changes and modification may be made in the illustrated and described embodiment of this invention without departing from the spirit and scope of the invention and, therefore, the appended claims aim to cover all such changes and modifications as fall within the true spirit and scope of this invention.

I claim:

1. A tuning fork assembly comprising, a tuning fork having a base and tines, a drive coil, a pick-up coil, a metal housing for said coils, said housing comprising a pair of matching housing members of configuration to provide a cavity between said tines, said coils being disposed within said cavity and said cavity being filled with impregnating material, said housing members being rigidly secured to the base of said fork, said housing members holding said coils between the tines of said fork.

2. The invention according to claim 1, wherein the impregnating material is epoxy resin.

3. A tuning fork assembly comprising, a tuning fork having a base and tines, a drive coil, a pick-up coil, a

metal housing for said coils, said housing comprising a pair of matching housing members of configuration to provide a cavity between said tines, said coils being disposed within said cavity and said cavity being filled with impregnating material, said housing members being rigidly secured to the base of said fork, said housing members holding said coils disposed between said tines, a tuning fork assembly support ring comprising a pair of substan tially identical half-rings each fixed to the rear surface of one of said housing members adjacent the fork base with the ends of said half-rings aligned and closely ad jacent each other but not in contact, and means coopcrating with said support ring to resiliently mount said assembly.

4. A tuning fork assembly comprising, a tuning fork having a base and tines, a drive coil, a pick-up coil, a housing for said coils providing for rigid mounting of same, and an enclosing structure, said housing having a lower and upper channel portions, said lower channel portion being adapted to be rigidly mounted on said tuning fork base and said upper channel portion being adapted to hold said coils between said tines, and means to resiliently mount said housing and said tuning fork in the said enclosing structure, said resilient mounting means including a support ring rigidly mounted on said housing, and a resilient means positioned between said support ring and said enclosing structure.

5. A tuning fork assembly comprising, a tuning fork having a base and tines, a drive coil, a pick-up coil, 21 housing for said coils providing for rigid mounting of same, and an enclosing structure, said housing having a lower and upper channel portions, said lower channel portion being adapted to be rigidly mounted on said tuning fork base and said upper channel portion being adapted to hold said coils between said tines, and means to resiliently mount said housing and said tuning fork in the said enclosing structure, said resilient mounting means including a support ring rigidly mounted on said housing, a skirt means and a resilient means positioned between said support ring and said skirt means, said enclosing structure being adapted to fit over said skirt means.

6. A tuning fork assembly comprising, a tuning fork having a base and tines, a drive coil, a pick-up coil, a housing rigidly secured to said fork for said coils providing for rigid mounting of same, an enclosing structure and means to resiliently mount said housing in said enclosing structure, said resilient mounting means including a support mounted on said housing and a resilient means positioned between said support ring and said enclosing structure.

7. A tuning fork assembly comprising, a tuning fork having a base and tines, a drive coil having a core pin, a

pick-up coil having a core pin, a housing for said coils providing for rigid mounting of same, said housing having a lower and upper channel portions, said lower channel portion having flanges which extend from the channel sides in a direction parallel to the plane of the channel bottom, said flanges being adapted to be mounted on the tuning fork base, said upper channel portion having its sides indented to provide space for the core pins of said coils; an enclosing means; and means to resiliently mount said tuning fork in said enclosing means, said resilient mounting means including a support ring mounted on said housing, and a resilient means.

8. A tuning fork assembly comprising a tuning fork having a base and tines; a magnetic housing rigidly secured to said base, said housing including two sections connected to opposite sides of said base and extending inwardly between said tines, said sections being spaced rom one another at all points to provide an air gap there between; and a drive coil and a pick-up coil rigidly mounted in said housing and disposed between said tines, said magnetic housing and said tines providing magnetic flux paths for said coils.

9. A tuning fork assembly comprising, a tuning fork having a base and tines, a housing of magnetic material rigidly secured to said base, said housing including two sections connected to opposite sides of said base and extending inwardly between said tines, said sections being spaced from one another at all points to provide an air gap; a drive coil and a pick-up coil rigidly mounted in said housing and disposed between said tines; an enclosing means; and means resiliently mounting said housing and tuning fork in said enclosing means.

10. A tuning fork assembly comprising: a tuning fork having a base and tines; a housing of magnetic material rigidly secured to said base, said housing including two sections connected to opposite sides of said base and extending inwardly between said tines, said sections being spaced from one another at all points to provide an air gap; a drive coil and a pick-up coil rigidly mounted in said housing and disposed between said tines; an enclosing means; and means resiliently mounting said housing and tuning fork in said enclosing means, said mounting means including a resilient member interposed between said enclosing means and said housing.

11. A tuning fork assembly comprising, a tuning fork having a base and tines, a housing of magnetic material rigidly secured to said base said housing including two sections connected to opposite sides of said base and extending inwardly between said tines, said sections being spaced from. one another at all points to provide an air gap; a drive coil and a pick-up rigidly mounted in said housing and disposed between said tines: an enclosing means; cooperable positioning means abutting said housing means and said enclosing means and a resilient member disposed between said cooperable positioning means.

12. A tuning fork assembly comprising: a tuning fork having a base and tines; a housing of magnetic material rigidly secured to said base; said housing including two sections connected to opposite sides of said base and extending inwardly between said tines, said sections being spaced from one another at all points to provide an air gap; a drive coil and a pick-up coil rigidly mounted in said housing and disposed between said tines; an enclosing means; cooperable positioning means disposed in pre-determined positions abutting said housing means and said enclosing means; and a resilient member disposed between said cooperable positioning means, said cooperable positioning means including a ring secured to said housing and provided with recesses and means operably associated with said enclosing means providing inwardly extending protrusions aligned with said recesses, said resilient means being interposed between said recesses and said protrusions whereby said housing is resiliently held in a pre-determined position in said enclosing means.

References Cited in the file of this patent UNITED STATES PATENTS 1,906,985 Marrison May 2, 1933 2,147,492 Mead Feb. 14, 1939 2,574.l88 Murray Nov. 6, 1951 2,632,211 Trigg Mar. 29, 1953 2,678,360 Bellis May 11, 1954 2,707,234 Dostal Apr. 26, 1955 

